It is generally known that a high dark resistivity semiconductor can be transformed from semi-insulating state to a quasi metallic state almost instantaneously when it is illuminated by picosecond or femtosecond optical pulses. Due to this property, high resistivity semiconductors have been widely used as photoconductive optically activated switches (OAS) because they are able to provide fast risetime, low jitter and high power capabilities. These qualities have proved most advantageous in devices that produce high intensity microwaves. There are, however, many other applications for these switches, for example, time domain meteorology, counter measure devices and plasma generation and diagnostics.
Most of these applications involve kilovolt sequential wave form generation by pico second optoelectric switching, direct DC to RF conversions by impulse excitations of a resonant cavity or high speed optoelectric modulation of millimeter waves. One oft cited example of generating RF signals by laser illumination was described by Chang et al in "Direct DC to RF Conversion by Picosecond Optoelectric Switching", IEEE MTT-S International Microwave Symposium Digest, May 1984, wherein the laser illumination was accomplished in a three stage "frozen wave" configuration. From the Chang description, it is readily apparent that the switch used to discharge the energy store is a most critical component in the pulse power system and is generally considered the single limiting factor of the system. In applications requiring more than one switch, such as the Marx Generator or the Frozen Wave Generator described by Chang, there are three critical properties that the switches must exhibit. These are large voltage hold-off, large effective quantum efficiency and fast risetime.
One type of switch that has been used in microwave generation systems with the aforementioned qualities is a semi-insulating gallium arsenide device as disclosed in SIR H695, Weiner, et al, issued Oct. 3, 1989. The gallium arsenide material used in these devices has a very high resitivity that has the capability to withstand high voltages even without laser light illumination. With laser illumination, this device has relatively high ever, large amounts of optical energy are required to activate this switch. Due to this inefficiency, the means for generating the necessary optical energy is expensive and bulky.
As a modification of the Weiner et al optically activated switch, an optically activated gridded bulk silicon pin diode, as described in "Optically Activated Switch for MM Wave and Laser Transmitters", Final Report, US Army Contract DAAL01-85-C0421, SRI, Princeton, New Jersey, was developed to enhance the RF output in frozen wave generators. Since the development of this device it has been realized that although silicon switches provide several advantages such as relative low cost and ease of manufacture, gallium arsenide provides several performance advantages not capable in silicon switches.
One major advantage of using gallium arsenide in OAS devices is that it can be obtained with resistivities greater than 1X10.sup.7 ohm-centimeter. This quality eliminates the problem of thermal breakdown and permits the device to be DC biased, thus significantly simplifying the pulser circuitry. Another advantage is that semi-insulating galiium arsenide contains numerous impurities which give rise to high densities of mid-band energy levels. High carrier densities can, therefore, be generated with optical radiation that has photon energy below the band gap of gallium arsenile. This results in very long absorption lengths and permits bulk rather than surface conduction. A high current switch can thus be thick rather than wide and the illuminated area can be kept to a minimum.
Accordingly, it is an object of those who design optically activated switches to increase the resistivity of the device during the off-state, while being able to activate the switch at relatively low energy levels and to provide increases in risetime and voltage capability. The invention described herein addresses these objectives.